Real-Time Monitoring (RTM) of chemical mechanical planarization (CMP) processes is currently a subject of great interest and active development. Also known as in situ monitoring or end-point detection, RTM compensates for process variations by automatically adjusting the polishing time for each polishing run. The result is improved process stability, better centering of the process on a desired target, and a reduced need for operator intervention. In addition to its enabling role in end-point detection, Real-time monitoring provides a wealth of data on physical characteristics of the polisher during operation. As will be described, these RTM data are valuable for understanding fundamental aspects of the polishing process, for identifying unusual conditions which indicate a need for unscheduled maintenance of the equipment, and for tuning the polishing process, among other benefits.
Many methods for performing RTM of CMP processes have been proposed. The techniques which have received the most attention use three different types of signals: optical reflectance, motor current, and polishing pad temperature. Other methods have also been explored, such as the use of vibrations. All of these different methods employ a signal which monitors the progress of a single polishing run in real time and provides a characteristic triggering feature used to halt the polish step of the recipe. By adjusting the polishing time, RTM compensates for variations in the polisher's removal rate, including for example long-term drifts due to polishing pad wear. Likewise, RTM can compensate for fluctuations in film thickness of incoming wafers caused by variations in the deposition process.
Unfortunately, the processes, methods, and physical structures for accurate RTM have not been completely satisfactory. Particularly lacking have been structures and methods that are useful for multi-head CMP machines, especially when one or more motors are shared between the several heads.